1. Field of the Invention
The present invention relates to a photo detector.
2. Description of the Prior Art
Flat panel displays are widely employed in various consumer products such as liquid crystal displays, liquid crystal televisions, plasma televisions, cellular phones, personal digital assistants (PDA), digital cameras, display panel of portable game machines, and touch panel of automatic transaction machines. Hence, how to increase the color and brightness of the flat panel display has become an important task. Most of today's displays, especially in various touch panels, include photo detectors for detecting ambient lights. By having color and brightness of the display would adjust according to ambient lights, consumers would perceive a much better visual experience.
Referring to FIG. 1, FIG. 1 illustrates a schematic view of a touch panel according to the prior art. As shown in FIG. 1, a conventional touch panel typically includes a top substrate 12, a bottom substrate 14, a black matrix layer 38 disposed on a portion of the top substrate 12, and at least a photo detector disposed on the bottom substrate 14. The photo detector is composed of a thin film transistor 18 and a photo diode 16. The photo diode 16 includes a p-type doping region 20, an n-type doping region 22, and an intrinsic region 24 disposed between the p-type doping region 20 and the n-type doping region 22. The thin film transistor 18 disposed adjacent to the photo diode 16 includes source and drain regions 26, lightly doped source and drain regions 28, an intrinsic region (not shown) disposed between the lightly doped source and drain regions 28, a gate dielectric layer 30, a gate 32, an interlayer dielectric layer 34 disposed on the source and drain regions 26 and the lightly doped source and drain regions 28, and two electrodes 36 electrically connected to the source and drain regions 26. Additionally, the gate dielectric layer 30 and the interlayer dielectric layer 34 are disposed to cover the photo diode 16.
Typically, the photo detector disposed on the bottom substrate changes the voltage of the storage capacitor when a finger reflects the light emitted by the light source of the backlight module. The relationship between the electrical current of the photo detector and the magnitude of the photo sensitivity is therefore a critical criterion to determine the effectiveness of a touch panel. However, most of the photo detectors in today's touch panels either lack sensitivity to ambient lights or have insufficient photo-electrical current. The cause for these problems is usually a result of using polysilicon semiconductor materials having uniform crystal lattice structure to fabricate the p-type doping region 20, the n-type doping region 22, the intrinsic region 24, the source and drain region 26s, the lightly doped source and drain regions 28, and the intrinsic region (not shown) disposed between the lightly doped source and drain regions 28. Despite the fact that the material made of polysilicon semiconductor has the advantage of having high speed for electron mobility and strong electrical stability, it still has the risk of causing aforementioned problems. Hence, how to improve the current fabrication to effectively increase the photosensitivity and photo-electrical current of photo detector has become an important task.